1.1. Field of the Invention
The present invention concerns high quality gallium nitride wafers suitable for the subsequent growth of efficient devices structures and a method of manufacture thereof.
1.2. Description of the State of the Art
Blue-Violet laser diodes (LDs) based on GaInN MQWs cw operating at room temperature were demonstrated in late 1995. The active structure was grown on sapphire by Metal Organic Vapour Phase Epitaxy (Jpn. J. Appl. Phys, 35, L74(1996)). However these first LDs with threading dislocations (TDs) densities on the GaN/sapphire substrate between 108 and 109 cm−2 suffer from degradation. The operating lifetime of these blue-violet laser diodes could reach 10000 hours when the density of dislocations in the GaN wafers used for the fabrication of LDs structures went below 107 cm−2. These low dislocations densities have been indeed reached using the Epitaxial Lateral Overgrowth (ELO) technology. The ELO technology can be described as follows: first, a few μm thick GaN layer is grown on sapphire or 6H—SiC. Next a dielectric (SiO2 or SiN) mask is deposited using well-established technologies like CVD or PECVD. Using standard photolithographic techniques, a set of parallel stripes, separated by window areas, is opened in the mask. During the initial regrowth, either in MOVPE (T. S. Zheleva, O.-H. Nam, M. D. Bremser, R. F. Davis, Appl. Phys., 71, 2472 (1997)) or HVPE [A. Sakai, H. Sunakawa, and A. Usui, Appl. Phys. Lett. 71, 2259 (1997).] or even sublimation growth, (S. Kurai, K. Nishino, S. Sakal, Jpn. J. Appl. Phys., 36, L184(1997)) selective area epitaxy is achieved. This means that the subsequent growth is initiated in the windows without any nucleation on the dielectric mask. Under proper conditions and once the GaN growing film reaches in the stripes, the mask level, epitaxial lateral growth over the mask starts and finally leads to a full coalescence and to a smooth surface suitable for device fabrication. The basic idea is that this technique may lead to a filtering of the defects: above the windows, the microstructure of the underlying GaN template is reproduced, whereas the laterally grown material (over the mask) is defect free. The masked areas stop the propagation of threading dislocations that arise from the template, since lateral growth proceeds from TD free vertical facets.
Currently, two main ELO technologies exist: the simpler one involves a single growth step on the striped opening. In this one-step-ELO (1S-ELO), growth in the opening remains in registry with the GaN template underneath (coherent part), whereas GaN over the mask extends laterally (wings) (FIG. 2). In this method, however, some parts of the surface remain highly defective (coherent part above the openings and coalescence boundaries). This makes the technology of LDs on ELO also complicated since device structures have to be fabricated on the good part of the template over layer and thereby the yield of fabrication is low.
Conversely, in the two-step-ELO (2S-ELO) process (FIG. 3), the growth conditions of the first step are monitored to obtain triangular stripes. This technique is well described for example in U.S. Pat. No. 6,325,850. Inside these stripes, the threading dislocations arising from the templates are bent by 90° when they encounter the inclined lateral facet. In the second step, the growth conditions are modified to achieve full coalescence. In this two-step-ELO, only the coalescence boundaries are defective. In the 2S-ELO technology, TDs densities are reduced to about 107 cm−2. Indeed, the bending of dislocations at 90° is a key feature which in the 2S-ELO reduces the TDs density beyond the simple blocking by the mask. When the dislocations meet the {11-22} lateral facet, their line is submitted to two kinds of forces, one acts to keep the line so normal to the surface, whereas the second term acts to align the dislocation with its Burgers vector.